1. Field Of The Invention
The present invention relates to a connecting unit for ribbon-joined optical fibers and to a method of making the same.
2. Description Of The Prior Art
To connect two optical cables of the "ribbon type" to each other, that is cables in which several optical fibers are disposed parallel to each other and united together by a single plastics covering so as to form a ribbon and one or more ribbons are joined to form the cable, it is necessary to dispose each optical fiber of a ribbon forming one cable in alignment with the corresponding fiber of a ribbon forming the other cable, so as to allow light to pass from one fiber to the other while minimizing dispersions and attenuations of the transmitted signal resulting from faults in fiber alignment.
In order to achieve such alignment simultaneously in all fibers forming the ribbon, the end of the ribbon itself is conveniently fitted into a rigid body, referred to as a connector, which keeps the fibers in a geometrically definite position. Thus, two connectors forming a pair are disposed and held in a confronting relationship and so aligned that the respective fibers can take the correct position for forming the optical connection.
Due to the requirements for connections which achieve the best alignment between all the fibers of the ribbon in order to minimize the attenuation of the light signal when passing through the connection, it is necessary to establish very reduced tolerances as regards possible coaxial faults or misalignments between the fibers of each interconnected pair and, as a result, very reduced tolerances in the sizes and positions of the housings for the fibers themselves in the connectors. In particular, by way of example, to connect ribbons made of single-mode fibers in which the diameter .phi. of the cladding of each fiber is 125 .mu.m and the mode diameter is equal to 9.5 .mu.m, the coaxial faults or misalignments of the axis of a fiber in a connector related to the axis of the corresponding fiber in the facing connector must not be higher than one .mu.m; so that in most cases the signal attenuation at the connection is lower than 1 dB, which is deemed to be the maximum acceptable loss value in the connection.
Making connectors having such high accuracy requirements is quite a delicate operation, taking particularly into account the fact that it is necessary to produce a great number of connectors to be matched, while ensuring the same qualitative alignment value to all of them.
Connectors are known in which the optical fibers are housed inside the grooves of a plate made of a crystalline material which are obtained during several steps by localized etching, at positions defined by protection templates. Therefore, in order to achieve the very high accuracy in size required for the housing grooves in a plate, the position and shape of which directly establishes the axis position of the fiber contained therein, particularly delicate and expensive working processes are required in producing such plates.
Also known (see European Patent Application EP 0 241 724) are optical connectors with coupling plugs which include a base plate provided with grooves to receive the optical fibers and guide plugs to which a flat plate of smaller sizes is fastened. The plates define holes between each other and the guide plugs and fibers are subsequently introduced into the holes.
In order to make the fit of the fibers in the respective holes possible, the base plate has an uncovered portion the grooves of which represent a guide for the introduction of the fibers. These connectors are made of a hard and brittle material, such as crystalline silicon or ceramic and the grooves are formed by removal of material by means of very precise grinding machines which ensure the requested tolerance values, necessary for the optical coupling of the fibers.
In order to make connectors of this kind particular working techniques and machinery to be used for producing each individual base plate are required. In addition the plates can exhibit differences in sizes resulting from the precision limits offered by the machines used for their production, because it is particularly complicated and expensive to keep these machines within the admissible tolerance values for an acceptable optical coupling.
Furthermore, in connectors in accordance with the aforementioned patent application, by coupling a base plate to a flat plate elements provided with holes in which the fibers must be subsequently inserted are provided, which means that a certain clearance between the fibers and the related holes, as well as between the plugs and the related holes must always be available. This clearance which is necessary for introducing the fibers into the holes, in particular when the operation is carried out in the field, is however to the detriment of the precision in the optical alignment at the connection point and adds to the other inexactitudes in size, some of which have already been mentioned above.
Also known are connectors described in U.S. Pat. No. 3,864,018 in which the connectors consist of identical plates provided with several parallel grooves coupled to each other so as to clamp the interposed optical fibers housed in the grooves and keep them in the desired geometrical position. In this structure, however, the coupling is provided between two connectors located at the ends of respective optical fiber ribbons through outer alignment elements in contact with the plate surfaces opposite those clamping the optical fibers. As a result, fibers between two coupled connectors are subjected to an imperfect alignment due to inexactitudes in the plate thickness which can be hardly avoided and which add to the other working inexactitudes of the grooves. As a result, the structure in accordance with this patent is only suitable for connectors which do not need too much accuracy, in the case of multimode fibers, for example.